--- title: "How to add 1D General Weirs" slug: "how-to-add-1d-general-weirs" updated: 2026-10-16T18:26:12Z published: 2026-10-16T18:26:12Z canonical: "help.floodmodeller.com/how-to-add-1d-general-weirs" stale: true --- > ## Documentation Index > Fetch the complete documentation index at: https://help.floodmodeller.com/llms.txt > Use this file to discover all available pages before exploring further. # How to add 1D General Weirs Water levels are often influenced significantly by weirs within the watercourse. The General Weir is a general purpose unit for modelling a broad crested weir with a rectangular throat. By amending the input coefficients, it is also possible to model weirs with a parabolic or triangular control section. The unit models the weir in free or drowned mode with forward or reversed flow. To add a 1D General Weir: 1. In the **1D River Network** table, confirm you have a **River Section** at both the upstream and downstream toe of the weir. 2. If only one River Section exists, select the River Section in the **1D River Network**, Right-click and select **Copy**. Right-click again and select **Paste**. Rename the duplicated River Section with an appropriate node label. 3. Set the **Distance to next** value as **zero** within the upstream **River Section**. 4. From the **1D River Build** tab, select the **Weirs** dropdown menu and select **General Weir.** Manually place the **General Weir** unit between the upstream and downstream River Sections. 5. Copy the node labels of the upstream and downstream River Sections into the upstream and downstream labels of the **General Weir** unit. 6. Define the geometry of the weir by specifying the **Elevation of Crest**. Note that the weir upstand does not need to be explicitly entered into the General Weir unit. 7. Define the **Breadth of Crest**. This is the breadth of crest perpendicular to flow. For non-rectangular control sections the breadth should be set to 1.0 and the **Velocity Coefficient** input as normal. 8. Specify the **Discharge Coefficient** and **Velocity Coefficient** as required. Note that these coefficients are multipliers applied to the broad-crested weir equation. See [Weir](/v1/docs/weir) for details on the equations used. 9. Specify the **Modular Limit** and **Exponent** as required. Note that the default Exponent of 1.5 is for modelling a rectangular control section. For parabolic or triangular control sections the exponent should be changed to 2.0 and 2.5 respectively. 10. Your General Weir is now added to your river reach. Make sure to save the adjusted river network! Download the sample data used in the video to try it yourself:

General Weir.zip